The present invention relates to automatic adjustment of optical measurement equipment in order to position an object to be studied at the focal point of a focused beam.
Coated thin film disks are used in a variety of industries including the semiconductor and the magnetic hard disk industry. During manufacture and processing, such coated thin film disks require inspection for a variety of reasons. For example, coated thin film disks are routinely inspected to identify defects and particles on a surface of the disk so as to avoid use of the area near the defect. Coated thin film disks can also be inspected to identify topological variations on the surface. This type of inspection complements defect detection, for example, when evaluating a magnetic hard disk or semiconductor wafer fabrication process.
Many inspection applications for coated thin film disks involve use of focused optical beams, such as focused laser beams, to study the surface of the coated thin film disk. In focused optical beam techniques, it is typically desirable to place the surface of the coated thin film disk at the focal point of the beam. One current method for positioning the coated thin film disk at the focal point of the focused beam is manual calibration. Each time a coated thin film disk with a new thickness is measured the inspection system must be recalibrated to position the surface of the object at the focal point of the focused beam. Manual calibration is a time intensive task that reduces the throughput and productivity of a measurement tool.
The composition of the coated thin film disk provides an additional complication in focusing an optical beam on the surface of the disk. Thin film disks or wafers may be made from a wide variety of materials such as, aluminum, glass, silicon, gallium arsenide, lithium niobate, lithium tantalate, quartz or sapphire. Each of these materials has its own reflectivity characteristics. As a result, light signals reflected from these various materials will exhibit different properties. For example, a focused light beam reflected from a surface will exhibit a shift in the centroid of the reflected beam relative to the outer envelope of the beam that depends on the composition of the reflected surface. The shift in the centroid of a focused reflected beam depends on both the reflectivity versus angle of the material being investigated and the cone angle of the incident beam. An example of centroid shift is illustrated in FIGS. 1a-1c. FIG. 1a shows the beam intensity profile of an incident focused beam. The concentric circles in FIG. 1a schematically represent increasing beam intensity as one moves from the edge to the center of the beam. FIG. 1b shows the intensity profile for the focused beam after reflection from a first material. FIG. 1b shows that the centroid of the beam has shifted compared to the outer envelope of the beam. Because of the centroid shift, the maximum in beam intensity no longer corresponds to the center of the beam profile. As noted above, the amount of this centroid shift relative to the outer envelope of the beam will depend upon the optical reflectivity versus angle of the material under investigation as well as the cone angle of the incident optical beam. An example of this can be seen in FIG. 1c, which shows the intensity profile for a focused beam after reflection from a different material. This shifting of the centroid of a reflected focused beam as a function of optical material prevents one from using a simple bicell, quad cell or other position sensitive detector to determine the location of the beam and hence the focal position of the beam.
In addition to the problems associated with the centroid shift of a focused beam after reflection from a surface, some thin film disk materials are at least partially transparent to optical wavelengths. This creates the potential for reflections from one or more secondary surfaces of a coated thin film disk, such as interfaces between coating layers or the backside of a transparent wafer or disk. These reflections from secondary surfaces further complicate the problem of determining when a focused light signal is focused on a desired surface.
What is needed is a method that allows an optical inspection tool to automatically calibrate the position of a thin film disk relative to the focal point of a focused beam. The method should allow for automatic focusing of a beam on a thin film disk independent of the reflective properties of the material being investigated. The method should also be effective for both transparent and non-transparent thin film disks or wafers.
The present invention provides a method for automatically focusing a light signal on the surface of a thin film disk or wafer. In an embodiment, a light signal is passed through a focusing lens and directed toward a surface of a thin film disk or wafer. The surface of the thin film disk, wafer, or other substrate is initially positioned at a distance beyond the focal length of the focused beam. Light reflecting off the surface of the substrate is spatially filtered and then received by a position sensitive detector, such as a bicell or quadrant detector. The position sensitive detector is configured so that the received light will straddle at least two elements of the position sensitive detector when the light signal is focused on the surface of the substrate. An output signal based on the intensities received by the detector elements of the position sensitive detector is generated as the surface is moved closer to the source of the focused beam. This continues until the difference between the intensities received by the detector elements of the position sensitive detector exhibits a mathematical critical point, such as a minimum value. At the mathematical critical point the surface of the thin film disk is a known distance away from the focal point of the beam. The spacing between the focal point and the surface of the thin film disk is reduced by the known distance. As a result, the focal point of the focused optical beam will coincide with the surface of the thin film disk or wafer.
The present invention also provides a method for determining a calibration distance for an optical measurement tool or other apparatus having a focused light source. In an embodiment, a light signal is passed through a focusing lens and directed toward a surface of a thin film disk, wafer, or other substrate. The surface of the thin film disk, wafer, or other substrate is initially positioned at a distance beyond the focal length of the focused beam. Light reflecting off the surface of the substrate is spatially filtered and then received by a position sensitive detector, such as a bicell or quadrant detector. The position sensitive detector is configured so that the received light will straddle at least two elements of the position sensitive detector when the light signal is focused on the surface of the substrate. An output signal based on the intensities received by the detector elements of the position sensitive detector is generated as the surface is moved closer to the source of the focused beam. This continues until the difference between the intensities received by the detector elements of the position sensitive detector exhibits a mathematical critical point, such as a minimum value. The position of the focused light source relative to the substrate at this mathematical critical point corresponds to a first calibration position having a first distance between the focused light source and the substrate. The relative positions of the substrate and the focused light source are then adjusted until the light signal from the focused light source is focused on the surface of the substrate. This position corresponds to a second calibration position having a second distance between the focused light source and the substrate. The difference between the first and second distances corresponds to the calibration distance.